Invasive prenatal tests are potentially harmful to the mother and to the fetus. Therefore, there is a need for the development of noninvasive prenatal tests. Maternal blood can contain fetal cells (see e.g., U.S. Patent Application Publication No. 20080070792) and cell-free fetal DNA (see e.g., Huang et al. (2008), Methods in Molecular Biology, 444:203-208). While circulating fetal cells present an attractive target for non invasive prenatal diagnostics, particularly for the diagnosis of fetal sex and chromosomal abnormalities by simple karyotyping, the scarcity of intact fetal cells in the maternal circulation (around one cell per ml of maternal blood), low efficiency of enrichment (Bianchi et al., Am J Hum Genet. 61:822-829 [1997]) and difficulties with chromosomal analysis associated with abnormally dense nuclei in some cells (Babochkina et al., Haematologica 90:740-745 [2005]), have favored research on cell-free DNA.
The establishment of the concentrations of cell-free fetal DNA (cfDNA) in maternal plasma in healthy pregnant women has formed the platform on which fetal DNA abnormalities in pregnancy-associated disorders can be studied. The finding of a gradual increase in fetal DNA concentration in maternal serum as gestation progresses has been shown to precede complications associated with preterm labor. A five-fold increase in fetal DNA concentration has also been found in the serum obtained from women affected by preeclampsia. Other pregnancy-related disorders that have been linked to an elevated concentration of cfDNA include hyperemesis gravidarum (severe morning sickness), invasive placentation (in which the placenta contacts the maternal bloodstream), intrauterine growth restriction, feto-maternal haemorrhage and polyhydramnios. (Wright C. F. and Burton H., Human Reproduction Update 15(1):139-151 [2009]).
Quantitative analysis of cell free DNA by real-time PCR strategies has also indicated that the concentrations of circulatory fetal DNA are increased in pregnancies with fetal aneuploidies, most notably trisomy 21 (Lo et al., Clin Chem 45:1747-1751 [1999]). However, the fraction of fetal DNA in maternal cell-free plasma DNA is usually determined by comparing the amount of fetal-specific locus (such as the SRY locus on chromosome Y in male pregnancies) to that of a locus on any autosome that is common to both the mother and the fetus by using quantitative real-time PCR (Dahllan et al., Lancet 369:474-481 [2007]; Li et al., Clin Chem 1002-1011 [2004]; Fan et al., Proc Natl Acad Sci 105:16266-16271 [2008]).
Thus, there is a need for additional methods that would enable the determination of the fraction of fetal nucleic acid in both male and female pregnancies.
The method of the invention fulfills the need in providing the means to determine fetal fraction that is independent of the gender of the fetus. The method can be applied for determining simultaneously the presence or absence of a chromosomal aneuploidy or other copy number variation, and may be used in conjunction with may known methods that are used to determine aneuploidies in maternal sample.